The purpose of this circuit found in an old Elektor is to light a lower power lamp when the bigger one burns out. The loads depend on the triacs.
Using SCRs, this circuit from a 1967 General Electric manual allows full wave load control with SCRs....
This interesting circuit from a publication from the 1970s, measures the intensity of the sound...
This simple NPN and PNP transistor tester configuration can use the low current range of a common...